If a patient develops an atrial tachycardia crisis (also called "supra-ventricular tachycardia" or SVT), it is necessary to limit the ventricular synchronization to a given range. This is necessary to avoid too rapid a stimulation of the ventricle, which would be beyond a maximal frequency that is physiologically tolerable by the individual, the so-called "maximal frequency of ventricular stimulation" or "maximal synchronization frequency".
To achieve this goal, the cardiac pacemaker is provided with a fallback mode of functioning in which it tries to keep a certain synchronization beyond the maximal synchronization frequency (i.e., operation in a Wenkebach operating mode for determined time interval or number of cardiac cycles) and, if the atrial frequency continues to exceed the maximal synchronization frequency, it changes to an asynchronous mode of ventricular stimulation. In the asynchronous mode the ventricular stimulation frequency is typically reduced to a frequency that is lower than the maximal synchronization frequency.
As soon as the atrial frequency falls below the maximal synchronization frequency limit, a phase of progressive re-synchronization also known as "fallup" is started.
FR-A-2,544,989 and U.S. Pat. No. 5,226,415 describe such modes of functioning, applied in the case of a "double chamber" type pacemaker. Such a fallback and fallup mode of operating also are implemented in the commercial devices known as the models CHORUS, CHORUS II and CHORUS RM double chamber pulse generators, available from ELA Medical, Montrouge, France.
The present invention is directed to the particular case where such a fallback mode is incorporated in a cardiac pacemaker of the "triple chamber" type. As noted, triple chamber type pacemakers include, in addition to a ventricular electrode (generally a bipolar endocardial lead), two atrial electrodes with one atrial probe implanted on each of the two atria and with both atrial electrodes connected to a corresponding single input to the pacemaker by use of a Y connector.
The atrial electrode is thus a double electrode but, in contrast with a classic "bipolar" electrode having two electrodes where the distal and proximal terminals are distanced only a few millimeters, the two terminals of the double atrial electrode are relatively spaced further apart, for example, a typical distance on the order of 5 cm. The double atrial electrode is preferably two unipolar electrodes connected by a Y-connector.
Some pacemakers, e.g., the model CHORUS II devices, also are equipped with an ability to switch between operation a bipolar mode of detection and stimulation, and a monopolar (unipolar) mode of detection and stimulation. These modes are well known to persons of ordinary skill in the art, and such a switching can be typically realized by external programming. Further, the switching can apply individually to the atrial circuit as well as to the ventricular circuit of a double chamber pacemaker. Thus, a triple chamber pacemaker can be constructed of a double chamber pacemaker and the double atrial electrode.
The triple chamber cardiac pacemaker has been used in a relatively satisfactory manner for some years. They are useful in connection with patients having indications presenting an "inter-atrial block" sinusal disorder, in which there is a deficient signal propagation (insufficient or too long) from the right atrium to the left atrium.
Thus, if only one atria is stimulated (e.g., the right atrium, as occurs in the classic situation of the "double chamber" pacemaker), the other atrium (e.g., the left atrium), which is not stimulated, would receive the depolarization wave coming from the stimulated atrium, if at all, after an excessively long period. Sometimes the period is longer than the atrial-ventricular (AV) delay. Such a phenomenon can result in a stimulated contraction of ventricles occurring before draining of the left atrium, and therefore before the mitral valve has closed. This produces a counter-flow of blood from the ventricle to the left atrium and a diminution of the hemodynamic efficiency.
In addition, the electrical desynchronization of the two atria favors the appearance of tachyarrhythmia events.
Further, it has been recognized that the inter-atrial propagation delay period increases with the patient's effort. Therefore, an increase of the physiological activity of the patient apparently favors the risk of appearance of tachyarrhythmia and/or reduced hemodynamic efficiency.
The known "triple chamber" pacemakers operate by stimulating both the left and right atria simultaneously. This is done to avoid the appearance or the persistence of the cited drawbacks. Nevertheless, clinical studies have revealed a sometimes defective functioning of the fallback mode for some patients, with a period of desynchronization continuing even after the return of the atrial rhythm to below the maximal synchronization limit. Such disfunction, whose origin had not up until now been recognized to exist or understood, could even continue for so long that the fallback mode is de-activated. Consequently, the therapist is obliged to choose between a functioning of the type "triple chamber without fallback" or of the type "double chamber with fallback", and therefore, not be able to combine, with all necessary security guarantees, a functioning of the triple chamber stimulation with a fallback mode.